Ion beam assisted deposition (IBAD) is a process used to deposit thin, adherent coatings onto a surface of a substrate. In this process, the substrate is placed into a vacuum chamber and a vapor and an ion beam are simultaneously used to deposit the thin coating onto the substrate. Ion beam assisted deposition has been used to deposit many different materials, including, for example, zirconium dioxide.
Zirconium dioxide or zirconia has found application on orthopedic implants. Zirconia exhibits many of the preferred and desired attributes for use in vivo. For example, zirconia is biologically inert, hard, and can be formed as a smooth film on an articular surface of an orthopedic implant. Unfortunately, the use of zirconia is not without difficulty.
One difficulty is that zirconia, like many other oxide and nonoxide ceramics, has multiple phases. Zirconia has a monoclinic phase, a tetragonal phase, and a cubic phase. In particular, the difficulty with zirconia arises due to a volume expansion that a zirconia unit cell undergoes when it transforms from tetragonal to monoclinic. The volume expansion is approximately 4%. When zirconia transforms, the volume expansion results in an enormous stress increase within a product made of zirconia. In fact, the stresses that form in bulk components of pure zirconia will often cause spontaneous catastrophic failure of the component. In other applications, particularly in orthopedic implant applications, the phase transformation may result in surface roughening as isolated pockets of tetragonal zirconia transform into monoclinic zirconia, essentially causing the surface to buckle outward as it attempts to expand. Surface roughening on articular surfaces is unacceptable.
Therefore, what is needed in the art is a process for forming a ceramic layer where a particular crystal structure of the ceramic layer may be selected. In addition, what is needed in the art is a process that permits selection or adjustment of the relative amounts of the constituents according to a relationship between the constituents and the ceramic layer such that the ceramic layer is formed with a desired structure having a predetermined ratio of two or more crystal structures. Furthermore, what is needed is a process for forming a zirconia ceramic layer having a desired structure comprising two or more crystal phases, but that does not roughen due to tetragonal to monoclinic zirconia transformation.